In a port container terminal, an inland container terminal, and the like, containers are loaded and unloaded on and off ships, rail cars, and trailers by quay cranes and gantry cranes . There is a seismic isolated crane in which as a seismic countermeasure for such a quay crane, a seismic isolation device is disposed between a leg structure and a travel device of the crane (see Patent Document 1, for example).
FIG. 6 shows a crane provided with seismic isolation devices. This crane 1X includes: leg structures 20 each formed from a sea-side leg 21 and a land-side leg 22; and a boom 24 and a girder 25 to be supported by the leg structures 20. Moreover, a seismic isolation device 2X is provided between each leg structure 20 and each travel device 23. Here, reference numeral 26 denotes a loading-unloading device (a trolley), reference numeral 27 denotes a container cargo ship, and reference numeral 28 denotes a container. Meanwhile, an x-axis direction indicates a lateral movement direction (a sea-land direction) of the crane while z indicates a vertical direction thereof.
Next, loading and unloading actions of the crane 1X will be described. The crane 1X performs loading and unloading actions to hoist a container 28 loaded on the container cargo ship 27 with the trolley 26, and to load the container 28 onto a trailer (not shown) standing by at a quay. In the meantime, the crane 1X performs loading and unloading actions to load the container 28 from the trailer into the container cargo ship 27. Meanwhile, in the course of the loading and unloading actions, the crane 1X performs the loading and unloading actions while moving along the quay (in a direction towards the back side of the sheet of FIG. 6 or in a direction towards the front side of the sheet FIG. 6) by using the travel devices 23, and thus changing unloading and/or loading locations.
Next, actions of the crane 1X in the event of an earthquake will be described. In the event of an earthquake, a shear pin or the like that fixes each seismic isolation device 2X breaks, thereby activating the seismic isolation device 2X. The seismic isolation device 2X has an effect of isolating the crane 1X from jolts on the ground surface. The seismic isolation devices 2X are required to be able to support the weight of the crane and to be deformable in a horizontal direction (such as the lateral movement direction x).
The seismic isolation device 2X will be described with reference to FIG. 7 to FIG. 9. FIG. 7A illustrates a side view of the seismic isolation device 2X at ordinary times. The seismic isolation device 2X includes: rubber bearing (laminated rubber) 3 formed by alternately laminating rubber materials and steel plates; a top plate 5; and a bottom plate 6. FIG. 7B illustrates a state where a top plate-side projection and a bottom plate-side projection completely overlap each other in a plan view of the laminated rubber 3. In other words, the laminated rubber 3 supports the weight of the crane with a support region S illustrated as a shaded portion. Here, C denotes a center line of the seismic isolation device 2X. In the meantime, the planar diameter of the laminated rubber 3 is in a range from about 400 to 700 mm.
FIG. 8A illustrates a side view of the seismic isolation device 2X in the event of an earthquake. The laminated rubber 3 is deformed by an external force F1 (a seismic force). FIG. 8B illustrates a state where a top plate-side projection ST partially overlaps a bottom plate-side projection SB. This laminated rubber 3 supports the weight of the crane virtually with a shaded support region S. In other words, it is necessary to secure at least a predetermined area for the support region S in order to support the weight of the crane with the seismic isolation device 2X. Here, L1 denotes a slide length of the seismic isolation device 2X. The slide length L1 is about 300 mm at the maximum. The conventional quay crane 1X acquires the seismic isolation effect by using the above-described seismic isolation devices 2X. Here, CT denotes a top plate-side center line and CB denotes a bottom plate-side center line.
The revision of Port and Harbor Act of Japan in May, 2006 has changed seismic assessment standards for quays and cranes. As a consequence, in some locations, there is a case where a crane is required to absorb a horizontal deformation in the sea-land direction in an amount of about ±1000 mm. The crane 1X equipped with the above-described seismic isolation devices 2X has several problems in dealing with these standards.
First, when rubber materials with a small spring constant, i.e., soft rubber materials are used for increasing a horizontal displacement, the laminated rubber 3 has a problem that, even in the state where the top plate-side projection ST and the bottom plate-side projection SB completely overlap each other at ordinary times as shown in FIG. 7B, the laminated rubber has a low vertical load bearing capacity and cannot continue to fully support the weight of the crane.
Second, when a height of the laminated rubber is increased to deal with large horizontal displacement, the laminated rubber 3 has a problem that the top plate-side projection ST and the bottom plate-side projection SB are completely misaligned, and the laminated rubber 3 cannot support a vertical load attributed to the weight of the crane. This state will be described with reference to FIG. 9. FIG. 9A illustrates a side view of the seismic isolation device 2X in the event of a large-scale earthquake. FIG. 9B illustrates a state where the top plate-side projection ST and the bottom plate-side projection SB do not overlap at all. In the event of the large-scale earthquake, an external force F2 becomes large and a slide length L2 becomes large accordingly (300 mm or more, for example). As a consequence, no support region S is formed. For this reason, the seismic isolation device 2X cannot support the weight of the crane, and overturning moment M occurs in the seismic isolation device 2X. In other words, this seismic isolation device 2X cannot withstand the large-scale earthquake.
Third, when the diameter of the laminated rubber 3 is increased, the laminated rubber 3 has a problem of involving a wasteful design with a large vertical load bearing capacity that is more than necessary at ordinary times because the laminated rubber 3 is to be designed on the basis of a vertical load bearing capacity restriction in the event of an earthquake. FIG. 10 illustrates a seismic isolation device 2Y including laminated rubber 3Y with an increased diameter. This seismic isolation device 2Y can obtain a portion (the support region S) where the top plate-side projection ST and the bottom plate-side projection SB overlap each other even when a horizontal displacement (of the slide length L2) occurs in the event of a large-scale earthquake. Accordingly, the seismic isolation device 2Y can support the vertical load attributed to the weight of the crane, and no overturning moment occurs therein.
However, it is necessary to select the laminated rubber 3Y so that the laminated rubber 3Y can support the vertical load attributed to the weight of the crane only with the area of the portion (the support region S) where the top plate-side projection and the bottom plate-side projection overlap each other. Here, a vertical load bearing capacity at ordinary times, i.e., when the top plate-side projection ST and the bottom plate-side projection SB completely overlap each other, is about three to five times as large as a vertical load attributed to the weight of the crane. This involves an extremely wasteful design.
Fourth, even if the increase in diameter of the laminated rubber is realized, the seismic isolation device has a problem of an increase in size that leads to an increase in weight of the crane. This is attributed to increases in weight of the rubber materials and the steel plates associated with the increase in diameter of the laminated rubber. A crane has a strict restriction against the increase in weight due to a relation with the quay strength. Accordingly, even if the above-mentioned seismic isolation device is successfully formed, it is difficult to employ the seismic isolation device in a container terminal at a port or the like.